Google’s own services generate goo.gl short URLs (Google Maps generates https://maps.app.goo.gl/ URLs for sharing links to map locations), so I assume this shutdown only affects user-generated short URLs. Google’s original announcement doesn’t say as such, but it is carefully worded to specify that short URLs of the “https://goo.gl/* format” will be shut down.

Google’s probably trying to stop goo.gl URLs from being used for phishing, but doesn’t want to admit that publicly.

This actually makes the most logical sense to me, thank you for the idea. I don't agree with the way they're doing it of course but this probably is risk mitigation for them.